Sulfur is found naturally in crude oil and carries through into diesel and gasoline fuels unless specifically removed through distillation. As a result, diesel and gasoline fuels used in engines may contain sulfur in concentrations up to 3000 parts per million (ppm). At such high concentrations, sulfur provides high lubricity in fuel pumps and injector systems that deliver the fuel to the combustion chamber in an engine. However, fuel sulfur also causes polluting emissions, particularly SO2 and soot particles, and poisons the advanced emission-control and after treatment devices that are being developed to enable diesel engines to meet progressively more stringent emissions standards. Sulfur dioxide emissions are associated with environmental problems such as acid rain. However, when the current sulfur level is reduced in fuels, high friction and wear occur on sliding surfaces of fuel delivery systems and cause catastrophic failure.
Fuels with lower sulfur content have lower lubricity compared to those with higher sulfur content. Thus, low-sulfur diesel fuels do not provide sufficient lubricity for use in diesel engines, and the use of low-sulfur diesel fuels results in high friction and catastrophic wear of fuel pumps and injectors. When lubricity is compromised, wear increases in fuel injection systems, most of which were originally designed with the natural lubricating properties of traditional diesel fuel in mind. The lower lubricity of low-sulfur fuels poses significant problems for producers as well as for end-users of diesel fuels. Reduction in lubricity also contributes to a loss in useable power due to the increased friction the engine has to overcome. Because fuels with lower sulfur contents exhibit increased friction characteristics compared to fuels with higher sulfur contents, a perfectly tuned engine experiences a noticeable drop in efficiency when the fuel is changed from a high-sulfur fuel to a low-sulfur fuel. The typical diesel fuel currently used by trucks is a high-sulfur diesel fuel having a sulfur content of about 500 ppm. Low-sulfur diesel fuels have a sulfur content of approximately 140 ppm. Ultra low-sulfur diesel fuels have a sulfur content of 3 ppm. Fischer Tropsch fuels, the cleanest of all fuels, have a sulfur content of approximately zero. Because of its zero sulfur content, Fischer Tropsch fuel is an attractive diesel fuel, creating the least amount of pollution. Unfortunately, because it contains zero sulfur, it has no lubricity at all. Thus, Fischer Tropsch fuel causes the highest wear damage on sliding test samples. If it were used in today's engines, it would cause the instant failure of fuel pumps and injectors. Thus, it is not sufficient to simply reduce the sulfur content of fuels, because doing so would rob diesel fuels of their value as effective lubricants.
New mandates by the Environmental Protection Agency (EPA) call for the reduction of sulfur in diesel fuels to levels as low as 40 ppm in 2004 and to 15 ppm beginning Jun. 1, 2006. Such a move would quickly lead to the catastrophic failure of diesel fuel system components. The same requirements are also in place in Europe and Japan. The United States has been closely monitoring the use of low-sulfur fuels around the world. In Sweden and Canada, low-sulfur diesel fuels have been used for several years. Problems with increased wear have been encountered in both countries. The wholesale introduction of low-sulfur fuel in Sweden has had a disastrous effect on diesel engine operation. Swedish refiners are now using additives to prevent excessive wear in fuel injection systems, and their problems are apparently under control. Certain major Canadian refining companies are also adding lubricants before delivering low-sulfur fuels to customers.
The American Society of Tool and Manufacturing Engineers (ASTME), the Society of Automotive Engineers (SAE), and the International Organization for Standardization (ISO), have not yet set fuel lubricity specifications for supplying or testing low-sulfur fuels. Because of added costs, refiners are unlikely to consider supplying a pre-additized fuel before a specification has been set. Until the lubricity specification is written and followed, responsibility rests with diesel equipment end users to use fuel additives to maintain the reliability of their diesel engines.
A common approach to the problem of low-sulfur fuels has been to add lubricant compositions to fuels that reduce friction in internal combustion engines. Various patents disclose additives formulated as lubricating oils and blended into fuels. Alcohols are well known for their lubricity properties when included in lubricating oil formulations. Alcohols are also known for their water-scavenging characteristics when blended into fuels. The use of vicinal hydroxyl-containing alkyl carboxylates, such as the ester glycerol monooleate, have also found widespread use as lubricity additives or as components in lubricating oil compositions.
Borated lubrication compounds are well known lubrication additives for fuel compositions. Borated lubrication compounds are known to have high viscosity indices and favorable low temperature characteristics. Such boron-containing compounds are known to be non-corrosive to copper, to possess antioxidant and potential antifatigue characteristics, and to exhibit antiwear and high temperature dropping point properties for greases. Borated esters and hydrocarbyl vicinal diols have long been proposed as fuel or lubricant additives, especially as mixtures of long chain alcohols or hydroxyl-containing aliphatic, preferably alkyl, carboxylates. Borated lubrication compounds are generally obtained by synthetic methods known in the art. Typically, these borated lubrication compounds are prepared by reacting boric acid or boric oxide with appropriate aliphatic or alkoxylated compounds.
Borated derivatives of phosphorus are also known additives for liquid fuel or lubricant compositions. Such borated phosphorus derivatives include borated dihydrocarbyl hydrocarbylphosphonates. Borated phosphite additives may be synthesized by reacting dihydrocarbyl phosphites with such boron-containing compounds as boric oxide, metaborates, alkylborates or boric acid in the presence of a hydrocarbyl vicinal diol.
Organometallic boron-containing compounds are yet another class of fuel additives. In low-sulfur fuels, such organometallic compounds effect a lowering of the ignition temperature of exhaust particles in diesel engines equipped with an exhaust system particulate trap. Organometallic compounds contain a metal capable of forming a complex with an organic compound. Useful metals for use in such compounds include Na, K, Mg, Ca, Sr, Ba, Ti, Zr, V, Cr, Ni, Mn, Fe, Co, Cu, Zn, B, Pb, and Sb. Borated versions of such organometallic complexes are derived or synthesized from both aliphatic and heterocyclic organic compounds.
Although various patents describe boron-containing additives that provide lubricity to fuel compositions, all the conventional additives are based on compositions that require prior synthesis before addition to the fuel. Some, such as phosphates and amines, require complex formulations and lengthy preparation, and therefore are not cost effective as fuel ingredients. These synthetics have not readily been taken up to replace sulfur in fuel compositions.
In terms of cost and effectiveness, the synthetics are impractical for several reasons. First, large amounts of additives are needed in order to achieve the same level of lubricity that a sulfur concentration of 500 ppm can provide in fuels. In addition, some of the current additives are “one shot” or “point-of-use” additives. These have to be added to the fuel tank at refills because they cannot easily be incorporated into the distillation processes in refiners. Other additives may fail when fuel injectors begin to operate at high pressures, such as 30,000 psi, because higher pressures mean smaller clearances between an injector's plunger and barrel, which results in more opportunity for engine wear. These higher pressures will soon be required by the EPA as part of the more advanced emission control technologies. Finally, the current additives may harm metallic or plastic fuel system components by causing corrosion and producing deposits in the long run.
Thus, a need remains for a readily available ingredient that can be easily and simply combined with low-sulfur fuel compositions to provide an additive that is inexpensive, non-toxic, and confers enhanced lubricity to low-sulfur fuels.